The present invention relates to compositions useful as reflectors. In particular, it relates to polymeric reflectors, especially parabolic reflectors, or optical functions.
It has been known for some time that a liquid in a container when spun at a constant rotational velocity assumes a parabolic cross-section. The forces on the liquid surface are constant normal to the surface (because of gravity) and vary with the square of the distance radially (centripetal acceleration).
A parabolic surface is ideally suited as an imaging optical device, being perfectly shaped for directing light to a focal point in front of the surface. This principal has been discussed a number of times over the last hundred years or so, and liquid metal techniques are used today by E. F. Borra to make primary mirrors for telescopes using mercury. These mirrors are of high quality, as befits their use in astronomical observations. The unfortunate disadvantages of liquid metal mirrors are obvious: they must be spinning continuously on an axis that is normal to a gravitational field, so they can observe only in one direction; and they cannot be used in space, where gravitational forces are all but eliminated.
The obvious extension of the Borra technique in liquid metal technology is to somehow solidify the liquid while it is spinning so as to preserve the parabolic shape. This process is known as spin casting and is often used to pre-form large, astronomical glass mirrors by slowly cooling a spinning dish of molten glass over a period of months. By spin casting glass mirrors, much of the surface modification, such as grinding or polishing, normally necessary when rough-forming the mirrors is avoided. The spun-cast surface is still imperfect, however, and requires polishing to obtain a dimensionally acceptable surface.
Spin casting has been applied recently to the casting of the world""s largest monolithic telescope mirror, one of the 8.4 meter mirrors for the Large Binocular Telescope (LBT). While spin casting of molten glass for telescope-grade reflectors is known, somewhat surprisingly, there are only a few publications describing attempts to spin cast polymeric reflectors. Furthermore, none of these attempts have produced a practical, optically precise plastic reflector.
There are no theoretical reasons why one cannot spin cast very accurate polymeric reflectors using thermosetting plastics. A rotating container of such polymeric material takes on the characteristic parabolic shape, as does any other liquid, and it will cure while spinning. Furthermore, there are a number of advantages to polymeric reflectors. Polymeric reflectors would be considerably lighter in weight than glass mirrorsxe2x80x94one-fourth the weight of similarly sized glass mirrors. Plastic composites are also much tougher than glass, being able to withstand much greater physical shocks without cracking or shattering. Both of these properties make plastic mirrors ideally suited for air or space-borne systems where weight is a critical factor and optical components need to withstand sudden accelerations and decelerations. Spin casting techniques also allow for the formation of parabolic optics of extreme curvature and short focal length. Such reflectors, indeed the total optical system, could be housed much more compactly than conventional, longer focal length optical systems, and the payload of the optical system would be reduced as well.
Additionally, very short focal length reflectors have the potential to replace many of the silicon lens systems used on planes and satellites today for infrared imaging. Short focal length, lightweight reflectors are also highly desirable as portable optical devices for use by individuals in the field. Polymeric, parabolic reflectors, mounted in an optical housing with appropriate secondary optics, for example, can be used as terrestrial telescopes or telephoto lenses.
However, in practice, making such mirrors is not a simple matter. Shrinkage and exotherms during polymerization create stresses that deform the surface of the plastic. Curing must be done under very controlled conditions of temperature and atmosphere for best homogeneity. Finally, not all polymers will produce reflectors having a high modulus and strength or a low coefficient of thermal expansion, all of which are necessary in any optical device.
Thus, there remains a need for an effective method for making high quality polymeric reflectors and parabolic reflectors in particular.
The present invention is a composition useful as polymeric reflectors, parabolic reflectors in particular, and a method for making the same, comprising the steps of building up the reflector in progressively thinner layers and then metalizing the final product to create a reflective surface. The present invention is an important advance over the prior art technology in that the reflecting surface of the composition is suitable for optical reflection before any of the aforementioned surface modification, such as polishing or grinding. For parabolic reflectors, there is the additional step of rotating a container at a constant rate, one selected to produce the desired parabolic profile, during the build up of layers. Each layer is cured before forming the next one and preferably while the rotation of the reflector assembly continues at the preselected rate.
In the initial layer, that layer which forms the backing of the reflector rather than the reflective surface, hollow glass microspheres and fibers may be added to the resin to reduce weight and add stiffness. Intermediate layers, being progressively thinner than the initial layer, can simply be made of resin, but other additives can be incorporated into the resin composition of each layer to achieve various goals such as the reduction of curing stresses. The uppermost layer is made of a specially formulated, high-purity polymer. Finally, the uppermost layer, when cured, is metalized via vacuum vapor deposition or other methods to provide a highly reflective surface. The entire process is done in a dust free, vibration-free environment.
An important feature of the invention is the use of progressively thinner layers to achieve the desired geometry in the cured product. The use of layers allows greater control over the final results because it minimizes the effects of shrinkage in the subsequent, thinner layers. Another important feature of the present invention, also related to the use of layers, is that not all layers need have the same composition. Various constituents can be added to each layer to reduce weight, add strength, lower cost, reduce stresses and produce a finer final layer.
Still another important feature of the present invention is that, because high quality optical reflectors can be made using the present technique of spin casting in progressively thinner layers, it is possible to quickly produce a reflector with a different focal length simply by changing the rate of rotation. A faster rotation rate will result in a shorter focal length. Traditionally, changing the focal length of cast polymeric reflectors involved making a new mold.